Time-frequency synchronization and frame number detection for dmb-t systems

ABSTRACT

A DMB-T receiver supports a single carrier (SC) form of modulation and a multi-carrier form of modulation such as orthogonal frequency division multiplexing (OFDM). Upon receiving a broadcast signal, the DMB-T receiver downconverts the received broadcast signal to a received base-band signal and determines frame timing synchronization from the received signal as a function of frame header mode  1  and frame header mode  3  by correlating groups of received symbols spaced at least two signal frames apart within a sample shift value in a range of plus or minus one.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/995,782, filed Sep. 28, 2007.

BACKGROUND OF THE INVENTION

The present invention generally relates to communications systems and,more particularly, to wireless systems, e.g., terrestrial broadcast,cellular, Wireless-Fidelity (Wi-Fi), satellite, etc.

Recently, the Digital Multimedia Broadcasting for Terrestrial Television(DMB-T) Standard was published for Digital Television (DTV) broadcastingin China (“Framing Structure, Channel Coding and Modulation for DigitalTelevision Terrestrial Broadcasting System,” NSPRC, August 2007). TheDMB-T standard specifies that a receiver support a single carrier (SC)modulation mode and a orthogonal frequency division multiplexing (OFDM)modulation mode (a multicarrier mode). For the single carrier mode,Quadrature Amplitude Modulated (QAM) symbols are transmitted directly.For the multicarrier mode, QAM symbols are modulated by an inverse DFT(discrete Fourier transform) operation. The DMB-T signal comprises ahierarchical frame structure with signal frames providing the basicbuilding block. A signal frame 10 is shown in FIG. 1. Signal frame 10comprises a frame header 11 and a frame body 12. Frame header 11 hasthree frame header modes of different lengths. As can be observed fromFIG. 1, these lengths are 420, 595 or 945 symbols. Frame body 12 conveys3780 symbols, of which 36 symbols are system information and 3744symbols are data. In a DMB-T system, a time-domain synchronous OFDM(TDS-OFDM) technique has been adopted. As such, the frame headersinclude pseudonoise (PN) sequences that serve as pilot signals and whichare also used as guard intervals instead of cyclic prefixes as found intypical OFDM transmission such as used in DVB-T (Terrestrial) (e.g., seeETSI EN 300 744 V1.4.1 (2001-01), Digital Video Broadcasting (DVB);Framing structure, channel coding and modulation for digital terrestrialtelevision) used in Europe.

SUMMARY OF THE INVENTION

As described above, a DMB-T signal comprises signal frames. A signalframe comprises a frame header and a frame body. There are three frameheader modes (modes) defined in the DMB-T Standard and the structure foreach mode is different. The frame headers of the different modes includepseudonoise (PN) sequences, which are inserted as guard intervalsinstead of cyclic prefixes as found in typical OFDM transmission such asthe above-mentioned DVB-T. Notwithstanding the different structures forthe different modes, and in accordance with the principles of theinvention, a receiver receives a signal for providing a sequence ofreceived symbols, the received signal having an associated signal framestructure; and synchronizes to frame timing in the received signal bycorrelating groups of received symbols spaced at least two signal framesapart within a sample shift value.

In an illustrative embodiment of the invention, the receiver is a DMB-Treceiver and supports a single carrier (SC) form of modulation and amulti-carrier form of modulation such as orthogonal frequency divisionmultiplexing (OFDM). Upon receiving a broadcast signal, the receiverdownconverts the received broadcast signal to a received base-bandsignal and determines frame timing synchronization from the receivedsignal as a function of frame header mode 1 and frame header mode 3 bycorrelating groups of received symbols spaced at least two signal framesapart within a sample shift value in a range of plus or minus one.

In view of the above, and as will be apparent from reading the detaileddescription, other embodiments and features are also possible and fallwithin the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show a DMB-T frames and DMB-T frame headers;

FIG. 3 shows illustrative data segments in accordance with theprinciples of the invention;

FIG. 4 shows an illustrative embodiment of an apparatus in accordancewith the principles of the invention;

FIG. 5 shows an illustrative flow chart in accordance with theprinciples of the invention;

FIG. 6 shows an illustrative embodiment of a receiver in accordance withthe principles of the invention; and

FIGS. 7-12 shows performance graphs for the various methods describedherein.

DETAILED DESCRIPTION

Other than the inventive concept, the elements shown in the figures arewell known and will not be described in detail. Also, familiarity withtelevision broadcasting, receivers and video encoding is assumed and isnot described in detail herein. For example, other than the inventiveconcept, familiarity with current and proposed recommendations for TVstandards such as NTSC (National Television Systems Committee), PAL(Phase Alternating Lines), SECAM (SEquential Couleur Avec Memoire), ATSC(Advanced Television Systems Committee), Chinese Digital TelevisionSystem (GB) 20600-2006 and networking, such as IEEE 802.16, 802.11h,etc., is assumed. Further information on DVB-T broadcast signals can befound in, e.g., ETSI EN 300 744 V1.4.1 (2001-01), Digital VideoBroadcasting (DVB); Framing structure, channel coding and modulation fordigital terrestrial television. Likewise, other than the inventiveconcept, transmission concepts such as eight-level vestigial sideband(8-VSB), Quadrature Amplitude Modulation (QAM), orthogonal frequencydivision multiplexing (OFDM) or coded OFDM (COFDM)) or discretemultitone (DMT), and receiver components such as a radio-frequency (RF)front-end, or receiver section, such as a low noise block, tuners, anddemodulators, correlators, leak integrators and squarers is assumed.Similarly, other than the inventive concept, formatting and encodingmethods (such as Moving Picture Expert Group (MPEG)-2 Systems Standard(ISO/IEC 13818-1)) for generating transport bit streams are well-knownand not described herein. It should also be noted that the inventiveconcept may be implemented using conventional programming techniques,which, as such, will not be described herein. Finally, like-numbers onthe figures represent similar elements.

As described earlier, there are three different frame header modes inDMB-T. These are shown in FIG. 2. Frame header mode 1 (11-1) comprises afront synchronization portion (21), a PN255 sequence portion (22) and arear synchronization portion (23). The front (21) and rear (23)synchronizations are cyclic extensions of the PN255 sequence (22). Thelength of the front synchronization is 82 symbols and the length of therear synchronization is 83 symbols. For frame header mode 1, a group of225 signal frames form a superframe (not shown) and these 225 frames usePN sequences generated by the same 8th-order linear shift register buthave different initial phases. Frame header mode 2 (11-2) comprises aPN595 sequence, which is truncated from a 10th-order maximum lengthsequence. For example, frame header mode 2 (11-2) is made up of thefirst 595 symbols from a PN sequence of length 1023. For frame headermode 2, a group of 216 signal frames form a superframe. Unlike frameheader mode 1, all frame headers contain the same PN595 sequence.Finally, frame header mode 3 (11-3) is similar to the structure of frameheader mode 1 (11-1). Frame header mode 3 comprises a frontsynchronization (41), a PN511 sequence (42) and a rear synchronization(43). The front (41) and rear (43) synchronizations are cyclicextensions of the PN511 sequence (42). The length of the frontsynchronization is 217 symbols and the length of the rearsynchronization is 217 symbols. For frame header mode 3, a group of 200signal frames form a superframe and these 200 frames use PN sequencesgenerated by the same 9th-order linear shift register having differentinitial phases. Notwithstanding the different structures for thedifferent modes, and in accordance with the principles of the invention,a receiver performs frame timing synchronization by correlating groupsof received symbols spaced at least two signal frames apart within asample shift value. In the following description, it is assumed that thereceiver has already determined the frame header mode in accordance withconventional techniques.

As noted above, the signal frame headers in a superframe use PNsequences which are generated by the same linear shift register but withdifferent initial phases for frame header modes 1 and 3. These PNsequences are cyclic shifts of each other. The initial phases of the PNsequences for each signal frame of a superframe are listed in NSPRC,“Framing Structure, Channel Coding and Modulation for Digital TelevisionTerrestrial Broadcasting System,” NSPRC, August 2007, mentioned earlier.After computer verification, we have found that the PN sequences havethe following structure. Let the PN sequence in the first signal framebe a reference PN sequence and P_(i)(l) be the PN sequence which iscyclically right shifted by l places relative to the reference PNsequence for frame header mode i. Then for frame header mode 1 thefollowing relationship holds:

$\begin{matrix}{{F_{1}(l)} = \left\{ \begin{matrix}{{P_{1}\left( {l/2} \right)},} & {{l = 0},2,\ldots \mspace{14mu},112} \\{{P_{1}\left( {254 - {\left( {l - 1} \right)/2}} \right)},} & {{l = 1},3,\ldots \mspace{14mu},111} \\{{F_{1}\left( {224 - l} \right)},} & {{l = 113},\ldots \mspace{14mu},224}\end{matrix} \right.} & (1)\end{matrix}$

where F₁(l) is the PN sequence which is used in the l^(th) signal framefor frame header mode 1. In similar fashion, for frame header mode 3 thefollowing relationship holds:

$\begin{matrix}{{F_{3}(l)} = \left\{ \begin{matrix}{{P_{3}\left( {l/2} \right)},} & {{l = 0},2,4,\ldots \mspace{14mu},100} \\{{P_{3}\left( {510 - {\left( {l - 1} \right)/2}} \right)},} & {{l = 1},3,5,\ldots \mspace{14mu},99} \\{{F_{3}\left( {200 - l} \right)},} & {{l = 101},102,\ldots \mspace{14mu},199}\end{matrix} \right.} & (2)\end{matrix}$

where F₃(l) is the PN sequence which is used in the l^(th) signal framefor frame header mode 3.

From the PN sequence structures given in equations (1) and (2), it isnoted that except for the two signal frames in the middle (these aresignal frames 111 and 113 in mode 1 and signal frames 99 and 101 in mode3), the cyclic shift of the PN sequence for every other signal frame iseither one place to the right or one place to the left. For the middletwo signal frames (again, these are signal frames 111 and 113 in mode 1and signal frames 99 and 101 in mode 3), the PN sequences which are oneframe next to them are either cyclically shifted by one place orunchanged. Therefore, for every other signal frame, the frame headershave at least L_(i)−1 repeated PN symbols, L_(i) is the length of theframe header for frame header mode i, where i=1, 3, (e.g., L₁=420symbols and L₃=495 symbols). In view of this, and in accordance with theprinciples of the invention, the PN Frame Header Correlation (FHC)function with respect to the timing instance m and s sample shift isdefined as:

$\begin{matrix}{{{{R_{fhc}\left\lbrack {m,s} \right\rbrack} = {\sum\limits_{k = 0}^{G_{i} - 2}{{r\left\lbrack {m + k} \right\rbrack} \cdot {r^{*}\left\lbrack {m + k + {2M_{i}} + s} \right\rbrack}}}};{i = 1}},{3;{s = {- 1}}},0,1} & (3)\end{matrix}$

where r[m] is the sampled received signal, G_(i) is the length of the PNsequence for frame header mode i, where i=1, 3, (e.g., G₁=255 symbolsand G₃=511 symbols); the parameter M_(i)=N+L_(i) is the length of asignal frame for frame header mode i, where i=1, 3, (e.g., M₁=4206symbols, M₃=4725 symbols). As such, the optimal frame timing, m₀, isgiven by:

$\begin{matrix}{m_{0} = {\arg \; {\max\limits_{0 \leq m \leq {M_{i} - 1}}\; {\max\limits_{{- 1} \leq s \leq 1}{{R_{fhc}\left\lbrack {m,s} \right\rbrack}}}}}} & (4)\end{matrix}$

It should be noted that it is assumed that the sample timing is equal tothe symbol timing. However, the inventive concept is not so limited andthe sample timing can different from the symbol timing. Equation (3) isconceptually illustrated in FIG. 3. A received signal, r, 80 is sampledfor providing a sequence of samples, e.g., sample 79 represents r[m] (atk=0). A sequence of samples 81 (from 0≦k≦(G_(i)−2)) are multiplied bycorresponding samples from a sequence of samples 82 (from 0≦k≦(G_(i)−2))located 2M_(i) samples away, i.e., at least two signal frames apart, andshifted forward or backward by s samples, where −l≦s≦l. In essence, asliding window correlation that is spaced at least two signal framesapart within a sample shift value. Once m₀ has been determined (equation(4)), the value of m₀ represents where the frame starts in symbols.

Referring now to FIG. 4, an illustrative embodiment of a device 50 inaccordance with the principles of the invention is shown. Device 50 isrepresentative of any processor-based platform, e.g., a PC, a server, aset-top box, a personal digital assistant (PDA), a cellular telephone, amobile digital television (DTV), a DTV, etc. In this regard, device 50includes one, or more, processors with associated memory (not shown) andalso comprises receiver 55. The latter receives a broadcast signal 1 viaan antenna (not shown)). For the purposes of this example, it is assumedthat broadcast signal 1 is representative of a digital television (DTV)service, i.e., a DTV transport stream, which includes video, audioand/or system information for at least one TV channel and that broadcastsignal 1 conveys this information using either a single carrier (SC)modulation or a multi-carrier modulation such as orthogonal frequencydivision multiplexing (OFDM). Illustratively, it is assumed the DTVservice is conveyed via DMB-T. However, the inventive concept is not solimited. Since, in this example, broadcast signal 1 uses at least threetypes of frame header modes, receiver 55 processes received broadcastsignal 1 in accordance with the principles of the invention forperforming frame timing synchronization. Subsequent to acquiring frametiming synchronization, receiver 55 further processes received broadcastsignal 1 to recover therefrom output signal 56 for application to aoutput device 60, which may, or may not, be a part of device 50 asrepresented in dashed-line form. In the context of this example, outputdevice 60 is a display that allows a user to view a selected TV program.

Referring now to FIG. 5, an illustrative flow chart in accordance withthe principles of the invention for use in device 50 is shown. In step205, receiver 55 performs frame header correlation (equation (3)) onsamples from a sampled received signal (e.g., refer back to FIG. 3).From this data, receiver 55 determines a peak value, m₀, (equation (4))in step 215. The value m₀ represents where the frame starts. Receiver 55signals frame timing synchronization has been achieved also in step 215.

Turning now to FIG. 6, an illustrative portion of receiver 55 is shown.Only that portion of receiver 55 relevant to the inventive concept isshown. Receiver 55 comprises down converter 110, demodulator 115, andframe timing synchronizer 120. In addition, receiver 55 is aprocessor-based system and includes one, or more, processors andassociated memory as represented by processor 190 and memory 195 shownin the form of dashed boxes in FIG. 6. In this context, computerprograms, or software, are stored in memory 195 for execution byprocessor 190, e.g., the above described flow chart of FIG. 5. Processor190 is representative of one, or more, stored-program control processorsand these do not have to be dedicated to the receiver function, e.g.,processor 190 may also control other functions of receiver 55. Forexample, if receiver 55 is a part of a larger device, processor 190 maycontrol other functions of this device. Memory 195 is representative ofany storage device, e.g., random-access memory (RAM), read-only memory(ROM), etc.; may be internal and/or external to receiver 55; and isvolatile and/or non-volatile as necessary.

Antenna 105 of FIG. 6 receives a broadcast signal and provides it toreceiver 55. In this example, antenna 105 provides received broadcastsignal 106 to down converter 110. Down converter 110 is representativeof the front-end processing of receiver 55 and includes, e.g., a tuner(not shown), etc., for tuning to and down converting received broad castsignal 106 to provide a base-band, or intermediate frequency (IF),received signal 111 for further processing by receiver 55. Receivedsignal 111 is applied to demodulator 115. In the context of DMB-T,demodulator 115 supports N modes of demodulation, where N>1. In thecontext of this example, N=2, where one demodulation mode is an OFDMmode and another demodulation mode is a SC mode. For the purposes ofthis example, it is assumed that received signal 111 is representativeof an OFDM signal using either frame header mode 1 or frame header mode3. Demodulator 115 demodulates received signal 111 to provideddemodulated signal 116, which is then further processed by receiver 55as known in the art (as represented by ellipses 130) to provide outputsignal 16. In accordance with the principles of the invention, frametiming synchronizer 120 processes data from demodulator 115 via signalpath 112 (as described above with respect to the flow chart of FIG. 5)to achieve frame timing synchronization for use by receiver 55. This isillustrated in FIG. 6, by signal 121, which signals that framing timingsynchronization has been achieved (e.g., step 215 of FIG. 5) for use byreceiver 55. It should be noted that although the various elements ofFIG. 6 are represented as single blocks, the invention is not solimited. For example, there may be separate demodulators, eachsupporting one, or more, types of demodulation.

It should be noted that the PN frame headers in modes 1 and 3 are alsodesigned to indicate signal frame numbers in DMB-T. In that regard,recognition of the PN sequence used in frame header modes 1 and 3 inaccordance with the principles of the invention can be used to detectthe frame number. In fact, use of equations (1) and (2) leads to a lowcomplexity frame number detector. Furthermore, this low complexity framenumber detector is immune to frequency offset.

Let S_(i)(l) be the number of cyclically right shifts from F_(i)(l) toF_(i)(l+1), according to (1) and (2), for i=1, 3, for frame header mode1 and frame header mode 3, respectively. In particular, for frame headermode 1,

$\begin{matrix}{{S_{1}(l)} = \left\{ \begin{matrix}{{\left( {- 1} \right)^{l + 1}\left( {l + 1} \right)},} & {0 \leq l \leq 111} \\{{\left( {- 1} \right)^{l + 1}\left( {224 - l} \right)},} & {112 \leq l \leq 224}\end{matrix} \right.} & (5)\end{matrix}$

and for frame header mode 3:

$\begin{matrix}{{S_{3}(l)} = \left\{ \begin{matrix}{{\left( {- 1} \right)^{l + 1}\left( {l + 1} \right)},} & {0 \leq l \leq 99} \\{{\left( {- 1} \right)^{l + 1}\left( {200 - l} \right)},} & {100 \leq l \leq 199.}\end{matrix} \right.} & (6)\end{matrix}$

It should be noted that the value of S_(i)(l) being negative indicates acyclic shift to the left. Thus, there is an unique cyclic shift fromF_(i)(l) to F_(i)(l+1). It should also be noted that there areL_(i)−∥S_(i)(l)∥ repeated symbols for frame header l and l+1. Fromequations (5) and (6) two look-up tables (LUTs) can be constructed. OneLUT for frame header mode 1 and the other LUT for frame header mode 3.In each LUT, each cyclic shift, s, is associated with a value of l, theframe number. For frame header mode 1, there are at least Z₁=308repeated symbols in two consecutive frame headers and for frame headermode 3, there are at least Z₃=845 repeated symbols in two consecutiveframe headers. Now, let R_(pnc) ^(R)[s] and R_(pnc) ^(L)[s] be the PNcorrelation functions with respect to s samples being shifted to theright and to the left:

$\begin{matrix}{{{{R_{pnc}^{R}\lbrack s\rbrack} = {\sum\limits_{k = 0}^{Z_{i} - 1}{{r\left\lbrack {m_{0} + k} \right\rbrack} \cdot {r^{*}\left\lbrack {m_{0} + k + M_{i} + s} \right\rbrack}}}};}{{R_{pnc}^{L}\lbrack s\rbrack} = {\sum\limits_{k = 0}^{Z_{i} - 1}{{r\left\lbrack {m_{0} + k + s} \right\rbrack} \cdot {{r^{*}\left\lbrack {m_{0} + k + M_{i}} \right\rbrack}.}}}}} & (7)\end{matrix}$

for i=1, 3. Then, let

$\begin{matrix}{{s_{R} = {\arg \; {\max\limits_{0 \leq s \leq 112}{{R_{pnc}^{R}\lbrack s\rbrack}}}}},{i = 1},3,{s_{L} = {\arg \; {\max\limits_{1 \leq s \leq 112}{{R_{pnc}^{L}\lbrack s\rbrack}}}}},{i = 1},3,} & (8)\end{matrix}$

Finally, the estimated cyclic shift of the PN sequence from F_(i)(l) toF_(i)(l+1) is given by:

$\begin{matrix}{\hat{s} = \left\{ \begin{matrix}{s_{R},} & {{{if}\mspace{14mu} {{R_{pnc}^{R}\left\lbrack s_{R} \right\rbrack}}} \geq {{R_{pnc}^{L}\left\lbrack s_{L} \right\rbrack}}} \\{{- s_{L}},} & {{{if}\mspace{14mu} {{R_{pnc}^{R}\left\lbrack s_{R} \right\rbrack}}} < {{{R_{pnc}^{L}\left\lbrack s_{L} \right\rbrack}}.}}\end{matrix} \right.} & (9)\end{matrix}$

The value of ŝ in equation (9) is then used to retrieve from theappropriate LUT the associated frame number, l.

As shown in FIG. 2, for frame header mode 1 and 3, a frame headercomprises a PN sequence and its cyclic extension. Thus, in frame headermode 1, the first 165 symbols of the frame header are repetitions of thelast 165 symbols of the frame header. Likewise, in frame header mode 3,the first 434 symbols of the frame header are repetitions of the last434 symbols of the frame header. As such, correlation between thesecyclic extensions can also be used to perform frame timingsynchronization. In particular, the Cyclic Extension Correlation (CEC)function with respect to the timing instance m is defined as:

$\begin{matrix}{{{R_{cec}\lbrack m\rbrack} = {\frac{1}{C_{i}}{\sum\limits_{k = 0}^{C_{i} - 1}{{r\left\lbrack {m + k} \right\rbrack} \cdot {r^{*}\left\lbrack {m + k + G_{i}} \right\rbrack}}}}},{i = 1},3.} & (10)\end{matrix}$

The parameter C₁=165 is the number of cyclic extended symbols, andG₁=255 is the length of the PN sequence for frame header mode 1.Similarly, the parameter C₃=434 is the number of cyclic extendedsymbols, and G₃=511 is the length of the PN sequence for frame headermode 3. Then, the optimal frame timing (sample index at the beginning ofa signal frame) is given by:

$\begin{matrix}{m_{1} = {\arg \; {\max\limits_{0 \leq m \leq {M_{i} - 1}}{{{R_{cec}\lbrack m\rbrack}}.}}}} & (11)\end{matrix}$

It should be noted that the presence of frequency offset causes a phaserotation proportional to the timing index. Thus, it is common toestimate the frequency offset Δf by:

$\begin{matrix}{{\Delta \; \hat{f}} = {{{Arg}\left( {R_{cec}\left\lbrack m_{1} \right\rbrack} \right)} \cdot \frac{f_{s}}{2\pi \; G_{i}}}} & (12)\end{matrix}$

where f_(s)=7.56 MHz is the symbol rate of the DMB-T system (e.g., seeF. Tufvesson, O. Edfors and M. Faulkner, “Time and FrequencySynchronization for OFDM Using PN-Sequence Preambles,” Proc. IEEE VTC,pp. 2203-2207, September 1999). The function Arg(•) is the mod-2π angleof the argument. It should be noted that because of the phase ambiguity,the frequency offset estimator given in equation (12) has itslimitations. For frame header mode 1, this estimator can estimatereliably when |Δf|<29647 Hz and for frame header mode 3, this estimatorcan estimate reliably when |Δf|≦14794 Hz.

The performances of the proposed frame timing synchronizers, frequencyoffset estimator and frame number detector have been demonstrated bycomputer simulations. The simulation environments are the additive whiteGaussian noise (AWGN) and multipath Rayleigh fading channel with rootmean square (RMS) delay spread equal to 1.24 μs (9.37 samples). For themultipath Rayleigh fading channel, the envelope of each single path isRayleigh distributed and the channel gains of each path are generated byJakes fading model (e.g., see P. Dent, E. G. Bottomley, and T. Croft,“Jakes Fading Model Revisited,” Electronics Letters, Vol. 29, No. 13,pp. 1162-1163, June 1993). FIGS. 7 and 8 show the standard deviation ofthe estimated timing for frame header mode 1 (FIG. 7) and frame headermode 3 (FIG. 8) under an AWGN environment. It can be observed that thestandard deviation is less than one sample when the SNR is around 5 dB.The frequency offset is set to 14 kHz in the simulation. As can be seenin FIG. 9 (for frame header mode 1) and FIG. 10 (for frame header mode3), the root mean square (RMS) residual frequency offset is close to 300Hz for frame header mode 1 and close to 100 Hz for frame header mode 3when the SNR is 0 dB. Finally, with respect to the frame header numberdetection, it can be observed in FIG. 11 (frame header mode 1) and FIG.12 (frame header mode 3) that the proposed algorithm yields excellentperformance, and that a frame number detection error is unlikely tooccur when the SNR is larger than −3 dB. It should be noted that theframe timing used in the frame number detector is obtained from the PNStiming synchronizer of equations (3) and (4).

As described above, and in accordance with the principles of theinvention, a frame timing synchronizer and a frame number detectorutilize PN patterns in frame header modes 1 and 3. A joint frame timingand frequency offset estimator based on the property of cyclic extensionin the frame headers was also described above. Simulation results showthat the performances of all proposed algorithms are excellent.Furthermore, the complexities of the proposed algorithms are very low,and hence, they can be easily applied in practical systems. Althoughdescribed in the context of an OFDM signal, the inventive concept isalso applicable to a single carrier signal. Further, it should berealized that conventional correlation techniques can be used for frameheader mode 2.

In view of the above, the foregoing merely illustrates the principles ofthe invention and it will thus be appreciated that those skilled in theart will be able to devise numerous alternative arrangements which,although not explicitly described herein, embody the principles of theinvention and are within its spirit and scope. For example, althoughillustrated in the context of separate functional elements, thesefunctional elements may be embodied in one, or more, integrated circuits(ICs). Further, the principles of the invention are applicable to othertypes of communications systems, e.g., satellite, Wireless-Fidelity(Wi-Fi), cellular, etc. Indeed, the inventive concept is also applicableto stationary or mobile receivers. It is therefore to be understood thatnumerous modifications may be made to the illustrative embodiments andthat other arrangements may be devised without departing from the spiritand scope of the present invention as defined by the appended claims.

1. A method for use in a receiver, the method comprising: receiving asignal for providing a sequence of received symbols, the received signalhaving an associated signal frame structure; and synchronizing to frametiming in the received signal by correlating groups of the receivedsymbols spaced at least two signal frames apart within a sample shiftvalue.
 2. The method of claim 1, wherein the signal frame structurecomprises a plurality of frame header modes, each frame header modehaving a pseudonoise sequence.
 3. The method of claim 2, wherein thesignal is a Digital Multimedia Broadcasting-Terrestrial televisionsignal and the synchronizing step is performed for frame header mode 1and frame header mode
 3. 4. The method of claim 1, wherein thesynchronizing step comprises: determining correlation values over groupsof the received symbols spaced at least two signal frames apart within asample shift value; and from the correlations values, determining a peakcorrelation value that is representative of where a signal frame startsin symbols.
 5. The method of claim 1, wherein the sample shift value iswithin a range of plus or minus one.
 6. Apparatus comprising: adownconverter for receiving a signal to provide a sequence of receivedsymbols, the received signal having an associated signal framestructure; and a processor for synchronizing to frame timing in thereceived signal by correlating groups of the received symbols spaced atleast two signal frames apart within a sample shift value.
 7. Theapparatus of claim 6, wherein the signal frame structure comprises aplurality of frame header modes, each frame header mode having apseudonoise sequence.
 8. The apparatus of claim 7, wherein the signal isa Digital Multimedia Broadcasting-Terrestrial television signal and theprocessor synchronizes to frame timing for frame header mode 1 and frameheader mode
 3. 9. The apparatus of claim 6, wherein the processor (a)determines correlation values over groups of the received symbols spacedat least two signal frames apart within a sample shift value; and (b)from the correlations values, determines a peak correlation value thatis representative of where a signal frame starts in symbols.
 10. Theapparatus of claim 6, wherein the sample shift value is within a rangeof plus or minus one.